In present wire based communication systems, information additional to the system data is typically queued up and transmitted in series with the system data as bandwidth permits. Such an approach utilizes valuable system bandwidth for the transmission of information additional to the system data, such as: control information (e.g. network router or switch flow control), signaling of status information, amplitude control, link parameter setup, equalization information, or network applications with very unsymmetrical data rates. Therefore, a need exists for the simultaneous transmission of additional information and system data over the same wired medium.
It would be further desirable for additional information to be transmitted simultaneously over the same wired medium to control the real-time operation of the transmitter and/or receiver to address certain problems in communication. For example, one problem of particular concern to communication systems having data rates in the Gbit/s range is that printed wires of a PCB back plane or the lossy transmission lines of a ceramic multi-chip carrier are introducing large amounts of unwanted ISI (intersymbol interference) in data transmission system. ISI reduces the timing margin of a transmission system by introducing a time shift in the zero (or reference level) crossings which is dependent on the history of transmitted bits. This is especially true for NRZ (non return to zero) transmission with its potentially large number of consecutive ones or zeros (so called long run length).
There are currently two methods used to reduce ISI introduced timing jitter. The first method is based on coding or scrambling of the data stream to guarantee that no long run lengths are occurring. The second method uses a best guess predistortion of the transmitted waveform which represents the data in order to correct the nonlinear behavior of the transmission media based on the knowledge of the polarity of previously transmitted bits. The first method either introduces coding overhead and thereby increases the bandwidth requirements of the transmission system, or in the case of scrambling, long run lengths can sometimes occur. The second method is based on a guess of the characteristics of the transmission channel. It will therefore result in sub-optimum solutions and must be programmatically adjusted for each data link. In the case of a high-density interconnection system, this is not practical due to the high number of links and the different lengths of transmission lines.
In addition, transmitter systems in which ISI introduced timing jitter is present sometimes use adaptive equalization in the receiver to correct for the nonlinear behavior of a transmission media. However, for transmission rates in the Gbit/s range, such equalization methods become very difficult to implement in a receiver because the ISI introduced timing jitter significantly reduces timing margin to the point where the receiver may no longer properly function.